1. Field of the Invention
The present invention relates to a magnetoelectric composite and, more particularly, to a layered composite in which the orientation of a piezoelectric material layer is controlled such that a <011> orientation is set to a thickness direction.
2. Description of the Related Art
A magnetoelectric (ME) effect is a material property and is widely found in composite structures comprising magnetostrictive and piezoelectric materials.
The ME effect means that, when any material having properties in response to both a magnetic field and an electric field is exposed to a magnetic field, an electric voltage is generated, whereas when the material is exposed to an electric field, it is magnetized. Thus, in order to impart an ME effect to a material, the material should essentially have both ferromagnetic, ferrimagnetic, or antiferromagnetic properties in response to an external magnetic field, and ferroelectric, ferroelectric or antiferroelectric properties in response to an external electric field.
In order to successfully commercialize magnetic-electric sensors, magnetic sensors, electric sensors, photoelectronic devices, microwave electronic devices, magnetic-electric or electric-magnetic transducers, etc., which are recently studied, development and research into materials having both ferromagnetic and ferroelectric properties at room temperature and having ME effects at room temperature or higher corresponding to the actual usage temperature of devices is ongoing.
A typical example of a material having both ferromagnetic and ferroelectric properties at room temperature is bismuth manganate (BiMnO3). However, the ferromagnetic phase transition temperature and the ferroelectric phase transition temperature (Tc) of bismuth manganate are about 100K and 450K, respectively, and this material has both ferromagnetic and ferroelectric properties only at 100K or more (N. A. Hill, K. M. Rabe, Physical Review B vol 59 pp 8759 (1999)). When a predetermined material has both ferromagnetic and ferroelectric properties only in such a very low temperature range, it cannot be applied to a variety of devices useful at room temperature, making it impossible to achieve commercialization. On the other hand, yttrium manganate (YMnO3), which has antiferromagnetic and ferroelectric properties, has been developed as a material similar to bismuth manganate, but the antiferromagnetic phase transition temperature and the ferroelectric phase transition temperature of yttrium manganate are 70˜130K and 570˜990K, respectively, and this material has both antiferromagnetic and ferroelectric properties only at a temperature equal to or lower than 70˜130K, making it impossible to achieve commercialization, as in condensers using bismuth manganate (A. Filippetti, N. A. Hill, Journal of Magnetism and Magnetic Materials vol 236 pp 176 (2001)). On the other hand, in the case of a condenser composed of Bi4Ti3O12 having a layered perovskite structure, only ferroelectric properties are exhibited at room temperature, and ferromagnetic properties are not, and thus it cannot be applied to devices requiring both ferromagnetic and ferroelectric properties.
Since the 1890s, many attempts have been made to develop homogeneous materials having magnetoelectric effects, and thereby homogeneous materials such as Cr2O3, Pb(Fe1/2Nb1/2)O3, BaMeO4 (Me=Mn, Fe, Co, Ni), Cr2BeO4, BiFeO3, etc., have been found to have magnetoelectric effects (G. Smolenskii and V. A. Ioffe, Colloque International du Magnetisme, Communication No 711958; G. A. Smolenskii and I. E. Chupis, Problems in Solid State Physics (Mir Publishers, Moscow, 1984; I. H. Ismailzade, V. I. Nesternko, F. A. Mirishli, and P. G. Rustamov, Phys. Status Solidi 57 99 (1980)). However, such materials are inappropriate for use in actual devices because the magnetoelectric coefficient is very low to the level of 0.001˜0.02 volt/cm·Oe and the temperatures at which magnetoelectric effects are shown are mostly as low as 0° C. or less.
In order to increase the very low magnetoelectric coefficient of single-phase materials as mentioned above, research has been conducted on developing composite materials having high magnetoelectric coefficients at room temperature or higher by mixing a material in response to a magnetic field with a material in response to an electric field. A typical composite material, which exhibits an magnetoelectric effect at room temperature or higher, is exemplified by Terfenol-D (a magnetostrictive metal in response to a magnetic field)/PZT (a ferroelectric oxide in response to an electric field)/Terfenol-D.
The Terfenol-D/PZT/Terfenol-D composite material is very unfavorable in terms of price because Terfenol-D is an expensive rare earth metal compound, and the magnetoelectric effect of the Terfenol-D/PZT/Terfenol-D composite material may considerably vary depending on the adhesion properties.
Also, the maximum magnetoelectric coefficient of the Terfenol-D/PZT/Terfenol-D composite material is about 10 volt/cm·Oe, which is higher than the magnetoelectric coefficients of single-phase magnetoelectric materials developed to date, but has to be further increased so that such a composite material can be applied to actual devices (J. G. Wan, J.-M. Liu, H. L. W. Chand et al. Journal of Applied Physics, Vol 93, No 12, pp 9916˜9919 (2003); Jungho Ryu, Shashank Priya, Kenji Uchino, and Hyoun-Ee Kim, Journal of the American Ceramic Society Vol 84, No 12, pp 2905˜2908 (2001)).
FIG. 1 illustrates a voltage generation principle of a conventional ME (MagnetoEletric) particulate composite, FIG. 2 illustrates the maximum magnetoelectric coefficient (αME) and the structure of the conventional ME particulate composite, FIG. 3 illustrates a voltage generation principle of an ME layered composite, and FIG. 4 illustrates the Terfenol-D/PZT/Terfenol-D composite. The layered composite of FIG. 4 is configured such that the piezoelectric material whose orientation is not taken into consideration because of the use of the piezoelectric ceramic PZT is layered with the metal magnetostrictive material Terfenol-D.
As illustrated in FIGS. 1 to 4, when a magnetic field is applied, the magnetostrictive material is deformed, and voltage is generated from the piezoelectric material due to generation of stress resulting from such deformation. As mentioned above, in the case of the Ni or Co-ferrite/PZT particulate composite, the maximum magnetoelectric coefficient (αME) is about 100 mV/cm·Oe. The layered composite such as Terfenol-D/PZT/Terfenol-D has a maximum magnetoelectric coefficient (αME) of about 10 volt/cm·Oe, which is increased by about 100 times compared to that of the ME particulate composite, not enough to apply it to actual devices.